Method of casting metals



Dec. 16, 1958 B. M. LORING METHOD OF CASTING METALS Filed March 16, 1951 v INVENTOR BLAK E M. LOR NG My ATTORNEY Elite The present invention relates to improvements in the art of casting materials, more particularly metals.

This application is a continuation-in-part of'my copending application Serial No. 59,738, filed November 12, 1948, now abandoned.

It is known that metal castings possess a coarse grain structure when the molten metal is allowed to cool down slowly in undisturbed fashion in the mold. The coarse grain structure is due fundamentally to a condition of zonal cooling in the hot body of metal, the outer zone or region cooling relatively quite rapidly to form a shell, followed by a stepwise inward solidification of the metal at a progressively slower rate. Comparatively few crystal nuclei are formed throughout the body of metal under these conditions and crystal growth is favored by the prolonged existence of an inner body of still molten metal. The crystal growth in extensive regions is of columnar or .dendritic form frequently extending along the line of the thermal gradients from the outer shell to the axis of the ingot, and being more pronounced the nearer the axis of the ingot.

It is also known to apply agitation of various types to the molten metal while in the mold in order to improve the structure of the casting, desirably to obtain a fine structure therein. Proposed procedures have included direct agitation by movement of the molten metal itself and indirect agitation by movement of the mold.

It is an object of the present invention to provide a method for producing improved castings of metal and materials of like solidification mechanism. It is another object to provide certain metal alloy castings of fine grain structure. It is a further object to provide an improved method of agitating molten material in the mold which does not involve movement of the mold or direct contact of the molten material with a force instrument required for the agitation. Other objects will appear from the description of the invention.

According to the method'of the present invention the molten material to be solidified is provided in .the mold with an overlying layer of a highly viscous lag through which is transmitted the force required to bring the molten material into the desired motion during the cooling period.

I have found that when the molten material to be solidified is forced to flow in the mold in a direction across the thermal gradient lines and by consequence across the line of the coincident dendritic crystal growth from the side walls of the mold, the formation of crystal nuclei in the metal is increased and the growth of the nuclei retarded to an extent that castings of a fine grain structure can be obtained without the aid of special nucleation assistants. In other words, I have found that the grain structure of metals and materials of like solidification mechanism can 'be improved when the molten material is given an up and down, or back and forth, oscillatory motion during cooling in the mold.

In the practice of the method of the invention, the slag layer is caused to oscillate by contact with a reciproice eating plunger or by other suitable force means. Preferably the frequency of oscillation is such as to cause the molten material in the mold to oscillate at the natural frequency of oscillation therefor. Once .this frequency has been set up, the plunger, for example, is moved in cadence therewith, and only sufiicient energy need be applied is is required to overcome damping out of the oscillations by the friction due to the side walls of the mold.

It is desirable in the case of the harder or tougher metals to pour the metal hot enough to permit beginning of agitation or forced flow before any substantial amount of solidification occurs, the object being to prevent or at least minimize the initiation of dendrites in the regions adjacent the walls of the mold. Proceeding in this way, the overall mechanical properties .of the metal ingots are improved by avoiding the zone of weakness existing at the juncture of the dendrite region and the equiaxed region. This zone of weakness is detrimental to the properties of the metal ingot even though the center metal possesses the desirable properties associated with fine grain size. Since these metal ingots are invariably intended for subsequent reduction treatment,

their overall mechanical strength, rather than that of the center section alone is important. Examples of metals with which this is particularly the case are high speed tool steels of the 18-41 type and stainless steel of the 2520 type (25% Cr, 20% Ni). Ingots of such metals are known to require long periods of homogenization to reduce segregation and, in addition, the reduction treatment must be done very carefully in order to avoid fracture of the ingot in breaking up the large dendrites. in the case of relatively soft metals, such as gun metal and bronzes, the application of force flow to the molten metal should be delayed until solidification has just begun and should be interrupted after a substantial but not complete solidification has taken place so as to avoid damaging the newly acquired fine grain structure of .the metal.

The method of the invention has the advantage that the applied force due to the elasticity of the slag layer tends to be distributed evenly to the body of metal or other molten material to be solidified. A further advantage due to the use of the slag layer is a greater retention of heat in the material whereby there is provided a lengthened period for Working of the material in the mold. Under the usual casting conditions the unprotected metal tends to form a crust or hard skin at the upper surface thereof, making working of the metal more difiicult. An indirect advantage of the method is a considerably smaller power input than is required where movement of the mold is used to agitate the metal.

The slag layer may be provided by disposing .on the molten material to be solidified a mixture of crushed glass and refractory in proportions such that when melted by the heat of the molten material there is obtained a slag of a highly viscous consistency. The slag remains sufficiently fluid at the interface between it and the hot molten material to permit it to act as an elastic or flexible blanket which rises and falls with the molten material. The composition of the slag will vary with the metal or other molten material to be cast but may be determined by simple tests within the skill of the art. For example, for casting of a high speed alloy steel having a composition of 18-4l type (18% W, 4% Cr, l% V), a mixture of about 50% by weight glass and 50% by weight refractory (BWK 3O brick) is suitable. Instead of forming the slag in situ, a crushed preformed slag of proper constitution may be employed. The thickness of the slag layer may be varied and will depend upon the instrument used to apply the force and the distance through which it is moved, the thickness in any event being that calculated to avoid piercing of the slag and entry of the molten material by the force instrument.

The invention will be more fully understood by reference to the following description. taken in conjunction with the accompanying drawing in which is shown apparatus suitable for the practice of the method of the invention and in which like numerals indicate like parts.

In the drawing:

Fig. 1 is a partly sectional view of one form of apparatus employing a two section mold, and

Fig. 2 is a like view of a modified form of apparatus employing a three section mold.

Practice of the method is illustrated with reference to the accompanying drawing. The hot molten metal 1 is poured into the mold whereupon the calculated quantity of a mixture of crushed glass and refractory is disposed on the surface of the molten metal and, after the highly viscous slag layer 3 has been formed, the

plunger 4 heated to the slag temperature is moved up and down in contact with the slag layer at a frequency such that the molten metal beneath is caused to oscillate at its natural frequency of oscillation. Oscillation of the metal is continued until solidification prevents further movement or it may be interrupted earlier depending on the results desired. The mold is of usual construction and may be formed of cast iron, steel or bakedsand. The plunger is made of preshaped refractory brick. Movement of the plunger may be made by hand in the case of small ingots and by a conventional form of reciprocating mechanism in the case of larger ingots.

Fig. 1 illustrates a preferred embodiment of the invention wherein a two section mold is employed for the casting. The hot molten metal partly fills both mold sections or risers 5 and 6 which are in communication by means of the bottom disposed passageway or conduit 7. The highly viscous slag layer 3 overlies the molten metal in each section of the mold and the plunger 4 is moved in one section to cause oscillation of the molten metal. Greater amplitude of displacement of the molten metal is provided and the natural frequency of oscillation of the molten metal is more easily obtained than with the use of a single section mold. The passageway or conduit 7 is so constructed .as to have a low heat transfer rate, for example, of a refractory having a low heat transfer coefficient whereby freezing of ,the metal therein does not occur until solidification of the ingots in the mold sections has been completed.

A further preferred embodiment of the invention is illustrated in Fig. 2, wherein a three section mold is used and a layer of slag 3 overlies the metal in each section. The mold sections or risers 8, 9 and 10 communicate through the botom disposed passageway 11 of a construction like that of the conduit 7 in Fig. 1. The operation of the method in this form of apparatus is similar to that illustrated in Fig. 1. As is apparent, a still greater amplitude of displacement of the molten metal is possible with this form of apparatus.

The method of the invention is applicable to the treatment of ingots of varying sizes. While it can be applied with valuable results to the casting of the softer metals such as bronzes and gun metal, it finds particularly valuable application in the casting of ingots of the harder, tougher steels, such as stainless steels of high chromium content c. g., 18% Cr8% Ni and 25% Cr20% Ni steels and high speed tool steels such as the well known 1841 steel (18% W4% Crl%V). Ingots of the harder, tougher steels cast by the method of the present invention can be rolled hot or cold with a substantial saving in time and power costs over those cast by the well-known quiescent method.

The method of the present invention is not limited in its application to improvement in metal castings but may be applied with beneficial results to the casting of nonmetallic materials which have a solidification mechanism similar to that of metals. For example, molten sulfur and molten silver nitrate can be cast following the method of the present invention with results very similar to those produced in metal ingots.

While I have described certain specific embodiments of the invention therein, the foregoing description is not intended by way of limitation since various modifications may be made in the practice of the invention without departing from the spirit or scope thereof.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalities thereon or thereafter.

What is claimed is:

A method of producing a metal casting of improved grain structure which comprises disposing a layer of highly viscous slag over the molten metal in each riser of a plural riser mold which communicates at the bottom of the mold for flow of the metal from one riser to the next and during cooling of the metal applying a force directly to the layer of slag in one of the risers at a frequency such as to cause the molten metal to oscillate in the mold at its natural frequency of oscillation.

References Cited in the file of this patent UNITED STATES PATENTS 

